. The strength of materials; a text-book for engineers and architects. by bending itbackwards and forwards, and steel wire in the process ofdrawing becomes very hard indeed. K ?5 (1 ^/ k f^ I ^ ^^ ^ ,^ c / 20 c^y ^ ^l/ / b h 15 e 10 5 o 5 10 o 5 (h) /o Fig. 26.—Overstrain. Recovery of Elastic Limit from Overstrain.—As in-dicated above, the elastic limit slowly recovers its originalvalue after it has been allowed to rest for a few hours; itthen will increase as the time of rest is extended and BEHAVIOUR OF MATERIALS UNDER TEST 59 ultimately gets above its final value and gets near to itsnew yie
. The strength of materials; a text-book for engineers and architects. by bending itbackwards and forwards, and steel wire in the process ofdrawing becomes very hard indeed. K ?5 (1 ^/ k f^ I ^ ^^ ^ ,^ c / 20 c^y ^ ^l/ / b h 15 e 10 5 o 5 10 o 5 (h) /o Fig. 26.—Overstrain. Recovery of Elastic Limit from Overstrain.—As in-dicated above, the elastic limit slowly recovers its originalvalue after it has been allowed to rest for a few hours; itthen will increase as the time of rest is extended and BEHAVIOUR OF MATERIALS UNDER TEST 59 ultimately gets above its final value and gets near to itsnew yield point. Mr. Muir * has shown that the temperature of boiling watergives an almost immediate recovery of the elastic limit tonear the new yield point which will be as high as if thematerial had been allowed to rest for several days. Hardening by Quenching.—The hardening effect of over-strain is not the same as that effected by heating the metalto a high temperature and quickly cooling by has the effect of making the metal very brittle, and if). Fig. 27. there is practically no yield point, the specimen breakingoff short with practically no extension. Mechanical Hysteresis .—If a specimen of ductile materialis loaded up beyond the elastic limit and the load is takenoff slowly and the strains noted for descending loads,the stress-strain diagram for descending loads will be foundnot to coincide with that for the ascending load, the twocurves forming a loop as indicated in Fig. 27; this, byanalogy with magnetic hysteresis, is called a mechanicalhysteresis loop. In experiments of this kind great care isnecessary to eliminate errors of the instrument on the return,but many experimenters have found similar results wellwithin the elastic limit. Very careful experiments byMr. Bairstow, however, at the National Physical Labora-tory,! suggest that this phenomenon does not occur unless* Phil. Trans. Roy. Soc, 1889. t ^^id., vol. 210. 60 THE STRENGTH OF MATERIALS the
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